Photochemical and Time-Resolved Spectroscopic Studies of
Dirhodium Hydrogen-Generating Species
by
David James Krodel
B.A. Chemistry
Northwestern University, 2000
SUBMITTED TO THE DEPARTMENT OF CHEMISTRY IN PARTIAL FULFILLMENT OF
THE DEGREE OF
MASTER OF SCIENCE IN CHEMISTRY
AT THE
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
FEBRUARY 2003
© 2003 Massachusetts Institute of Technology. All rights reserved.
Signature of Author_____________________________________________________________
Department of Chemistry
January 17, 2003
Certified by ___________________________________________________________________
Daniel G. Nocera
Professor of Chemistry
Thesis Supervisor
Accepted by___________________________________________________________________
Robert W. Field
Chairman, Departmental Committee on Graduate Students
Photochemical and Time-Resolved Spectroscopic Studies of
Dirhodium Hydrogen-Generating Species
by
David James Krodel
Submitted to the Department of Chemistry on January 17, 2003 in Partial
Fulfillment of the Requirements for the Degree of Master of Science in Chemistry
ABSTRACT
Photochemical and time-resolved spectroscopic studies of di- and mono-rhodium inorganic
complexes were undertaken to elucidate the mechanism of photocatalytic hydrogen generation
from hydrohalic acid solutions. Nanosecond transient absorption (NSTA) experiments on
solutions of Rh2(dfpma)3PPh3(CO) (dfpma = bis(difluorophosphino)methylamine) in THF and
methylene chloride revealed a long-lived (τ ~ 12 ms) intermediate species with an absorption
maximum at 395 nm. This intermediate was shown to be quenched by PMe3 following pseudofirst-order kinetics characterized by a 4.3 * 104 M-1 s-1 second-order rate constant. Preliminary
NSTA experiments undertaken in the presence of HCl showed behavior indicative of bulk
reaction of Rh2(dfpma)3PPh3(CO) with HCl yielding Rh2(dfpma)3PPh3Cl2. Transient absorption
experiments on Rh2(dfpma)3PPh3Cl2 revealed a long-lived (τ > 3 ms) transient species with an
absorption max at 400 nm. NSTA experiments undertaken with Rh2(dfpma)3PPh3Cl2 in the
presence of HCl showed no distinction from those performed in solvent alone. Steady-state
photolysis experiments showed that the presence of dissolved CO greatly increases the rate of
decomposition of Rh2(dfpma)3PPh3(CO) when irradiated. This coupled with NSTA data pointed
to the possibility of a ligand re-arranged intermediate during the photolysis of
Rh2(dfpma)3PPh3(CO). An improved synthesis of Rh2(dfpma)3Cl2 was found by allowing 1 eq.
[Rh(COD)Cl]2 to react with 3 equivalents of dfpma. Subsequent thermal chemistry showed that
Rh2(dfpma)3Cl2 did not react thermally with H2 or HCl. Photocatalytic studies showed that 2
days of irradiation in 0.1 M HCl/THF solution yielded only 3 – 5 eq. H2, as compared to 10 – 50
eq. of H2 when starting with Rh2(dfpma)3Cl4. This ruled out Rh2(dfpma)3Cl2 as the major
catalytic species in that process. Preliminary investigations of (PPh3)3RhCl as a mononuclear
catalyst capable of taking HCl to H2 and PPh3Cl2 in the presence of light determined the process
to be complex and low-yielding.
Thesis Supervisor: Daniel G. Nocera
Title: Professor of Chemistry
2
Acknowledgements
There are many people that I owe greatly for their friendship, support, advice and
guidance in the past few years, making this possible. The first of these people is Dan Nocera for
teaching me not only about chemistry but also much about myself. While the lessons were not
always enjoyable at the time, in the end I was always thankful for them. Following Dan, there is
the Nocera group who were great coworkers and friends. This group of wonderful scientists
provided clear insight into many problems and injected laughter into many days. In particular, I
thank Alan Heyduk for getting me started on this project and Steven Kaye who was an
undergraduate co-worker on the project during my final months. Both were of particular value to
the content of this thesis. I must mention my classmates Dave Manke and Bart Bartlett who
provided many great late-night conversations and arguments when the lab was otherwise silent.
Without their humor and camaraderie these years would have been much less fun.
A few friends from outside of the Nocera Lab kept me sane as well. Dave Lahr
introduced me to hockey and was always up for a night on the town. Bob and Emily Hefty I
thank for barbeques, Monday Night Football, and other events that gave us much needed social
outlets. My roommates, Brent Fisher, Bob Kennedy and Serhan Altunata, I thank for being good
friends and an ear to talk to when the day was done.
Bob Field I thank for a being a great professor in quantum mechanics and also for being
an incredibly understanding and supportive person when I was in desperate need of someone to
help me maneuver through this year of change. Ned Thomas I thank for taking a chance on me
and being so understanding of my final decision. Susan Brighton deserves a medal for being
advisor, councilor and mother for me and the many other graduate students in the Department of
Chemistry.
Scott Miller, taught me everything I know about lasers and transient absorption
spectroscopy. Scott always had time for helping others, especially me, with their experiments–
even when he had barely time enough for his own. That, together with the fact that he gave me a
place to live, puts him high on the list of people I owe big time.
Of all my co-workers and friends, I must acknowledge Chris and Michelle Chang for
their unwavering support both scientifically and emotionally. Their willingness to talk me
through many days of difficulty and uncertainty has led me directly to the path I now walk and I
am forever grateful.
My family, especially my parents, Jim and Anne, and my sisters, Meghan and Jennifer, I
thank for their love and support during my entire life and in particular during this very difficult
year. They are the foundation for everything I have done and anything I might achieve.
Lastly, I thank Nima Kudalkar, whose love and support has kept me going and believing
in myself through this year. Of all the exciting events and changes that happened this year, those
involving you will be forever in my heart and memory.
3
Table of Contents
Abstract .......................................................................................................................................... 2
Acknowledgements........................................................................................................................ 3
Table of Contents .......................................................................................................................... 4
List of Figures and Schemes......................................................................................................... 5
1. Background and Introduction.................................................................................................. 6
2. Nanosecond Transient Absorption Studies on the Low-Valent Cycle ............................... 10
2.1 Introduction ....................................................................................................................... 10
2.2 NSTA Studies of 1 and 2 in Solvent Only ........................................................................ 11
2.3 NSTA Studies of 2 with Trimethylphosphine................................................................... 14
2.4 NSTA Studies of 1 and 2 with HCl................................................................................... 15
2.5 Steady-state Bulk Photolysis Studies on 1 and 2 .............................................................. 16
2.6 Experimental ..................................................................................................................... 18
3. Photochemical and Thermal Studies on the High-Valent Cycle......................................... 21
3.1 Introduction ....................................................................................................................... 21
3.2 Thermal Synthesis of Rh2dfpma3Cl2, 4a........................................................................... 21
3.3 Performance of 4a as a Hydrogen-Generating Photocatalyst ........................................... 22
3.4 Subsequent Thermal and Photochemical Reactivity of 4.................................................. 22
3.5 Experimental ..................................................................................................................... 23
4. Studies of a Mononuclear Hydrogen-Generating Photocatalyst ........................................ 25
4.1 Introduction ....................................................................................................................... 25
4.2 General Observations of the Mononuclear System........................................................... 25
4.3 Variable Temperature NMR of the HCl Addition Step .................................................... 26
4.4 Determination of 31P Spin-Lattice Relaxation Times ....................................................... 27
4.5 Experimental ..................................................................................................................... 27
References .................................................................................................................................... 29
Curriculum Vitae.......................................................................................................................... 30
4
List of Figures and Schemes
Figure 1.1. Gray’s isocyano dirhodium complex ........................................................................... 6
Figure 1.2. Dulebohn’s homologous series of dirhodium complexes............................................ 7
Figure 2.1. Transient absorption spectrum of 1 excited at 355 nm after 1 µs.............................. 12
Figure 2.2. Ground-state absorption spectrum of 1 (X = Cl) ....................................................... 12
Figure 2.3. Transient absorption spectrum of 2 excited at 355 nm after 1 µs.............................. 13
Figure 2.4. Ground-state absorption spectrum of 2...................................................................... 13
Figure 2.5. Proposed identity of Rh20,I species............................................................................. 14
Figure 2.6. Single-wavelength kinetics trace of 2 at 410 nm (λexc = 355 nm)
for a) 50 eq. PMe3 and b) 100 eq. PMe3 ................................................................................ 15
Figure 2.7. Transient absorption spectrum of 2 in 500 eq. HCl at 1 µs and 1 ms........................ 15
Figure 2.8. Photolysis of 2 (λexc > 338 nm) in THF a) before and
b) after addition of 1 atm CO ................................................................................................ 17
Scheme 1.1. Low-valent photocatalytic cycle ................................................................................ 7
Scheme 1.2. High-valent photocatalytic cycle................................................................................ 8
Scheme 2.1. Proposed mechanism for photolytic decomposition of 2 in the presence of CO ..... 17
5
Chapter 1. Background and Introduction
The ability of Nature to convert solar energy into chemical energy has long intrigued
chemists. The efficient conversion of sunlight into energy-rich chemicals such as hydrogen,
oxygen, and simple sugars has been a goal of photochemistry for the past century.1 With other
sources of energy being depleted, becoming uneconomical, or unsafe, the quest for these sunlight
trapping processes becomes ever more urgent. For the past fifteen years, the Nocera group has
been focused on the realization of this goal through the photocatalytic activity of discrete,
molecular, binuclear transition metal species.
The genesis of this work could clearly be seen by examining the first attempts of
chemists to generate hydrogen photocatalytically in homogeneous solution. The early work of
Lehn, Sauvage, and co-workers in this field outlined the basic components necessary for such a
system.2,3 The use of highly reducing ruthenium (II) bipyridal excited states to photochemically
reduce species such as rhodium bipyridal complexes capable of reactions with mineral acids and
molecular hydrogen has been a cornerstone of this type of chemistry ever since. Drawbacks of
these systems, including the wasting of solar energy during the photoreduction of a catalytic
intermediate, led to the more aesthetically pleasing idea of the excitation of a single molecule
capable of catalyzing the conversion of solvated protons to molecular hydrogen. This concept
was first addressed by Harry Gray in his use of M—M singly2+
N
N
I
C
Rh
C
N
CN
N
N
I
C
Rh
C
dirhodium
diisocyanopropane
NC
C
bonded
C
N
species
(Figure
bridged
1.1)
and
by
similar
four
1,3-
ligands
to
stoichiometrically convert HX(aq) (X = Cl, Br) to H2 and the
oxidized metal complexes photochemically.4,5 The inability of
the complex to photoreduce the strongly bound halogens to
produce the diatomic halogen species and regenerate the catalytic
Figure 1.1. Gray’s isocyano
dirhodium complex
ground state precursor species prevented the realization of a true
photocatalytic
system
6
for
hydrogen
production
by
a
homogeneous
N
P
F3 P
Rh
P
0
P P
N N
P
P
Rh
N
0
PF3
F3P Rh
P
P
0
P
N
P
Rh
X
II
X
X P
P
II
Rh
X Rh
X
II
P
P
P P
N
N
P
N
molecular
N
P
species. Two possibilities for
X
this system’s shortcomings are
the lack of driving force for
Figure 1.2. Dulebohn’s homologous series of dirhodium
complexes (fluorines omitted from dfpma ligands for clarity, X =
Cl, Br).
the expulsion of halo ligands
and the lack of a cis geometry
for coordinated reductive elimination. Dulebohn and Nocera sought to rectify these issues by
utilizing a ligand that would cause a ground state disproportionation in the partial oxidized
dirhodium core.6,7 This stabilization of a Rh20,II species with the Nixon ligand8
bis(difluorophosphino)methyl amine (dfpma) and the isolation and interconversion between its
fully oxidized Rh2II,IIand fully reduced Rh20,0 forms engendered the idea that ground state
stabilization of the disproportionated species would encourage the complex to react in twoelectron redox steps. Moreover, these molecules exhibited the cis regiochemistry that would be
desired for a concerted reductive elimination of the halo ligands.9 It was ultimately shown by
Nocera and Heyduk that these compounds
N
P
Ph 3 P Rh 0
P P
N
– CO
P
Rh 0 CO
P P
N
were competent in the photoconversion of
hν
the presence of a sacrificial reductant (THF
hydrohalic acids to molecular hydrogen in
PR h 0 Rh 0
2Trap-X
hν U V
2Trap
P
N
P
or 1,3-butadiene) used to trap halogen
2 HX
radicals.10 This advance, while not an
hν Vis
energy conversion scheme (due to losses
H2
X
Ph 3 P Rh 0
Rh II X
P
P
P P
N N
through
sacrificial
reductants),
was
important as a means of producing
hydrogen, desirable for its use as a future
fuel and in fuel cells, from a non-fossil fuel
based source. In this work, two catalytic
Scheme 1.1. Low-valent photocatalytic cycle
7
cycles were identified. The first of which
made use of the Rh2
0,II
to
Rh20,0
N
X P
P X
X RhII RhII X
P
P
P P
N
N
redox couple
and was named the “low-valent” cycle. The
make use of the Rh2
II,II
to
Rh20,II
2Trap
H2
second, identified later, was presumed to
hνUV
redox couple
2Trap-X
2 HX
and designated the “high-valent” cycle.
Rh0RhI IX2
The bulk photochemistry and catalytic
ability
of
the
low-valent
cycle
Scheme 1.2. High-valent photocatalytic cycle.
were
thoroughly investigated by Heyduk. He observed specifically the photocatalytic reduction of a
0.1 M HCl/THF solution to produce molecular hydrogen by the action of Rh2dfpma3Cl2(PPh3), 1,
in the presence of white light (λ > 338 nm).10 This was found to occur for a period of 12 hours
culminating in a total of 125 turnovers with in
initial rate of 27 turnovers/hour. It was also shown
that
one
could
start
with
the
complex
Rh2dfpma3(CO)(PPh3), 2, to enter the same
photocatalytic cycle by photolabilization of the
carbonyl moiety. When either of these complexes
N
P
P
Ph3P Rh0
P
P
N
RhII
X
P
N
P
1
a, X = Cl; b, X = Br
N
P
X
Ph3P Rh0
P
P
Rh0
P P
N N
CO
P
2
was irradiated with UV light (λexc = 355 nm) in the presence of acid, a blue species (λmax = 580
nm) was observed which decomposed rapidly in the presence of room light (τ1/2 ~ 5 mins) to
give the spectrum of 1. This species was postulated to be a tetranuclear “0,I,I,0” dimer by
analogy with similar observations in Gray’s isocyano bridge system,11 although this spectrum
had not been independently reproduced by any thermally prepared compound. The possibility of
this species participating in, lying outside of, or running parallel to the catalytic cycle remained
unconfirmed.
Preliminary investigations into the high-valent cycle were also undertaken. For two
reasons this high-valent cycle appeared very promising. First, Heyduk had shown that the
8
quantum
N
X P
II
X Rh
P
Rh
X
II
X
N
X
0
P Rh
P
P P
N
P
P P
N
N
3
P
Rh
P
N
P
X
P
0
II
II
OC Rh
Rh
P
P
P
N
P
P
N N
4
a, X = Cl; b, X = Br
X
5
X
yield
of
the
photoreduction from this Rh2II,II
species to the Rh20,II was higher by
a order of magnitude than the
comparable transformation in the
low-valent cycle. Second, the high-
valent photoreduction could be accomplished at wavelengths further to the red. The Rh2II,II
precatalyst was confirmed to be a viable system by the observation of hydrogen evolution (20 –
50 turnovers) in a 0.1M HCl/THF solution by 3 when irradiated with white light (λ > 360 nm).12
Surprisingly, the photocatalytic process was characterized by the appearance of a previously
unobserved steady-state species. This intermediate was identified by the independent preparation
of Rh2dfpma3Cl2, 4a, via the removal CO from the complex Rh2dfpma3Cl2(CO), 5a, in vacuo. 4a
was identified by elemental analysis,
19
F NMR, and subsequent reactivity with σ-donating
ligands although no single crystals could be obtained for X-ray diffraction.
Because the use of a sacrificial reductant undermines the ultimate goal of solar energy
storage, more information about the mechanism of this process is needed to make educated
modifications to the photocatalytic species, maximizing the chemical energy stored by these light
harvesting complexes.
9
Chapter 2. Nanosecond Transient Absorption Studies on the Low-Valent Cycle
2.1 Introduction
With a working cycle for the photocatalytic production of molecular hydrogen in hand,
improving this process became the next goal. Three key improvements to the process would be:
1) increasing the wavelength of light used; 2) increasing the quantum efficiency of the
photoreductive step; and 3) eliminating the need for a halogen radical trap, a thermodynamic
sink in the process. Instead of addressing these issues directly through a survey method,
examining new systems and evaluating their performance, the approach taken was one of a
mechanistic study. By determining the mechanism of photocatalytic hydrogen generation and in
particular the crucial photoreductive step, insight into the design of new, more efficient systems
may be made. Specifically this information could be used to direct the sort of survey method that
could produce better systems for hydrogen producing and/or energy conversion applications.
Since general photochemical studies of the low-valent cycle had been undertaken, the knowledge
of the system was at a point to undertake real-time spectroscopic studies.
The chosen technique for elucidation of this mechanism was nanosecond transient
absorption spectroscopy (NSTA). The technique was well suited to mechanistic studies because
of its capability to observe non-isolable species on a time scale commensurate with that of
primary chemical reactions. The key principle of this technique was that of pump-probe. A laser
was used to photochemically excite the sample. After thus initiating the chemical reaction, a
white-light source was used to probe the absorption spectrum of the transient species produced in
real-time, observing events from 10 ns to 50 ms in duration. Both absorption spectra (~250-nm
window) at a given time after the initial pump pulse and single-wavelength kinetics of an
absorption feature could be obtained.
Initial estimates showed that transient absorption should be a viable method for observing
these transient species. Using the third harmonic of a Nd-YAG laser outputting 7 ns pulses (λexc
= 355 nm), a 2.5 mJ pulse would provide 7.4*10–9 Einsteins per pulse. Given a typical beam
10
diameter of 3 mm and a 2-mm path length quartz cell, this would introduce a local concentration
of ~5*10–4 Einsteins/L. Assuming a solution with an absorbance of 2 is anaylzed–corresponding
to a concentration of Rh compound (ε ~ 20,000) of ~0.5 mM–the number of photons available
would be roughly equal to the number of molecules in the path of the laser. Although Beer’s Law
does not hold in this limit, a rough estimation including the quantum yield of photoreduction of
the 0,II species (~10–4),13 showed that the concentration of transient species may be on the order
of 10–7 M. This would correspond to a ∆OD of 10–4 assuming that the molar absorbtivity of the
transient is similar to that of the starting materials. This is barely outside the limits of detection
of our nanosecond transient absorption setup. However, this is probably a worst case scenario as
initial-step quantum yields are likely to be higher then quantum yields corresponding to the
observed products of steady state photolysis. As transient absorption studies commenced this
proved to be the case as typical observed transients were found to have a ∆OD between 10–3 and
10–2.
The low-valent cycle could be approached from two photochemical starting points. It
could be entered starting from 1 using a photon to eliminate one or both halo ligands from the
coordination sphere and initiating reaction of the complex with HX. Alternatively, 2 could be
used to absorb a sacrificial photon (non-catalytic) to generate a previously unseen open
coordination site via expulsion of the CO from the coordination sphere that is then poised to
react with HX. With these two photo-initiated pathways, studies of the mechanism of
photocatalytic hydrogen production were begun.
2.2 NSTA Studies of 1 and 2 in Solvent Only
Two solvents were identified for use in the study of these photochemical processes, THF
and methylene chloride. THF was selected for its ability to serve as a halogen radical trap as was
shown by the photochemistry of the cycle.10 Methylene chloride was selected as a control solvent
because it does not trap halogen radicals, as previously shown,10 and it has very low absorption
at the excitation wavelength selected (355 nm). Benzene contributed too much fluorescence to be
11
useful as a solvent in these NSTA experiments. Initially, studies of the photochemical behavior
of 1 and 2, were undertaken in these two solvents. By examining the behavior of these species in
a minimally reactive chemical environment, it was hoped that the nature of the first step in each
photochemical process would be determined.
Transient absorption spectra of 1 in THF
6
and methylene chloride showed identical behavior.
4
A long-lived transient (τ > 3 ms) with a peak at 455
that the transient spectrum obtained upon irradiation
-3
∆OD/ 10
nm was observed (Figure 2.1). It should be noted
2
0
-2
of 1 was the same regardless of whether Br or Cl
-4
was the halo ligand with only a slight difference in
-6
400
the bleach as would correspond to the different
ground state spectra. Notable features of this
450
500
Wavelength/ nm
550
Figure 2.1. Transient absorption spectrum
of 1 excited at 355 nm after 1 µs.
spectrum were the bleach reaching a maximum
around 400 nm, which corresponded to the ground state absorption of 1. Due to the observed
bleach, the true maximum of this transient absorption feature was likely further to the blue than
455 nm. Since the transient was nowhere spectrally distinct from the bleach and the relative
molar absorptivities of the ground state and the transient were unknown, it was not possible to
determine the actual maximum by subtracting out
0.6
0.5
however, on the assumption that at 400 nm the
0.4
bleach is spectrally distinct from the positive
OD
the bleach from the spectrum. It could be estimated,
0.3
0.2
transient that the actual maximum could reside 10 –
20 nm to the blue of 455. This positive absorption
feature could be tentatively assigned to two species,
0.1
0.0
300
400
500
600
700
Wavelength/ nm
the two-electron oxidation product requiring the
expulsion of both halo ligands, or the one-electron
12
Figure 2.2. Ground-state
spectrum of 1 (X = Cl).
absorption
oxidation product requiring the initial expulsion of
8
only one ligand (Figure 2.5).
6
methylene chloride showed very different behavior
from that of 1. A transient absorption spectrum was
∆OD/ 10
-3
NSTA spectra of 2 in both THF and
4
2
0
recorded in THF having a transient peak at 395 nm
-2
and no observable bleach (Figure 2.3). The absence
400
of a bleach was consistent with the ground state
absorption spectrum of 2 (Figure 2.4) which had a
450
500
Wavelength/ nm
550
Figure 2.3. Transient absorption spectrum
of 2 excited at 355 nm after 1 µs.
maximum at 325 nm (outside the spectral window
of the experiment) and a small shoulder out to ~400
2.0
nm which would not be expected to be noticeable in
395 nm was long-lived (τ = 12 ms) and was very
different in shape from the transient peak observed
at 455 nm for 1. In methylene chloride, similar
behavior was observed to that in THF, excepting
after the experiment had been repeated a number of
OD
1.5
the presence of the observed transient. The peak at
1.0
0.5
300
400
500
600
Wavelength/ nm
Figure 2.4. Ground-state absorption
spectrum of 2.
times on the same sample in a given day. In these instances, a vestige of the peak at 460 nm
began appearing in the transient spectrum. This was indicative of bulk photolysis in the presence
of methylene chloride that led to 1 as a reaction product. This was further supported by steadystate photolytic experiments (vide infra). This indicated that the methylene chloride was acidic,
that photochemical activation of the rhodium complex led to reaction with methylene chloride
mimicking HX chemistry, or that photochemical activation of methylene chloride produced HX
in solution. Separate experiments using the same source of methylene chloride but performed by
others in the laboratory involving acid sensitive Zn porphyrins indicated that the solvent was not
13
very acidic before irradiation, as small traces of acid induce demetallation and a dramatic color
change from purple to red or green in these solutions.12
The fact that the transient spectra of 1 and 2 were not the same indicated different
primary reaction photoproducts of these two species. In the case of 2, the photochemical product
should be simply a coordinatively unsaturated dirhodium species. Ligand rearrangement may
have been involved but essentially the main photochemical process must be expulsion of CO to
give a Rh20,0 species. Similarly if 1 reacted by a two-electron reduction
process, it should have led to a similar spieces both structurally and
Ph3P Rh
spectrally. If instead, 1 was photoreduced by a one-electron process it
P
would be expected that this novel Rh20,I species (Figure 2.5) to be Figure
structurally and spectrally distinct from the valence symmetric species
N
P
identity of
P
0
P
N
X
I
Rh
P
N
P
2.5.
Proposed
Rh20,I
species.
obtained from 2. Furthermore, the fact that the transient obtained from irradiation of 2 was
further to the red than that from 1 was consistent with a ligand-to-metal charge transfer (LMCT)
band that would be present in a Rh20,I species containing a halo ligand, but would not be
consistent with a Rh20,0 species without oxidizing halo ligands.
2.3 NSTA Studies of 2 with Trimethylphosphine
The tentative assignments of the transient species needed to be bolstered by observing
their behavior in the presence of a different chemical environment. If indeed the transient
produced upon irradiation of 2 was the coordinatively unsaturated Rh20,0 species, the transient
should have been quenched by an environment rich in a strong σ-donating ligand such as a
phosphine. Phosphine was chosen above CO due to the inherent technical difficulties and
dangers associated with getting high concentrations of CO in solution (i.e. high pressures of CO
in sealed glassware). PMe3 was chosen after it was determined that PPh3 produced too much
fluorescence and contributed spectral features that could not be deconvoluted to the NSTA
spectra. The lifetime of the transient species was shown to decrease with increasing
concentration of PMe3 and this quenching process followed pseudo first-order kinetics due to the
14
7.5
τd = 1.4 ms
5.0
2.5
3
0
-3
8
-3
b)
∆OD /10
∆OD /10-3
a)
4
0
-4
6
τd = 0.77 ms
4
2
0.0
0
-2.5
-2
0.0
0.5
1.0
Time /ms
1.5
0
1
2
3
Time /ms
4
Figure 2.6. Single-wavelength kinetics trace of 2 at 410 nm (λexc = 355 nm) for a) 50 eq. PMe3 and b)
100 eq. PMe3.
large excesses of PMe3 used (50 and 100 eq. vs. 2). At 50 eq. of PMe3 (16 mM) it is shown that
the rate of decay is 700 s–1 and at 100 eq. (32 mM) the rate is 1300 s–1 (Figure 2.6). These
numbers give a second-order rate constants of 4.2 * 104 M–1 s–1 and 4.4 * 104 M–1 s–1 which
agree within the error of the experiment. This piece of evidence corroborates the hypothesis that
the transient species obtained upon photolysis of 2 is the Rh20,0 coordinatively unsaturated
species. With this assignment on surer ground preliminary NSTA studies on these two species in
the presence of HCl were undertaken.
2.4 NSTA Studies of 1 and 2 with HCl
20
In the first study of 1 with HCl, it was
produce any difference in the transient spectrum
at short or long times. It had been hoped that this
experiment might lead to variations in the
transient spectrum showing the next step after the
-3
∆OD /10
shown that 0.1M HCl (~ 500 eq. HCl) did not
15
1 ms
1 µs
10
5
0
-5
400
450
500
550
Wavelength (nm)
opening of a coordination site, but none was
found. The transient spectra observed upon
15
Figure 2.7. Transient absorption spectrum of
2 in 500 eq. HCl at 1 µs and 1 ms.
irradiation of 2 were more complex (Figure 2.7). At fast time scales (~ 100 ns – 1 µs), the
spectrum looked like the superposition of the transient spectra of 1 and 2 in THF without acid
present. This was similar to the instance where methylene chloride was used as solvent for 2 and
vestiges of the transient of 1 were seen in the NSTA spectrum, due to bulk photolysis involving
the acidic solvent. In this case, however, this behavior was expected, as bulk photolysis in HCl
was known to lead to 1 from the previously investigated photochemistry of the photocatalytic
cycle for H2.13 Attempts to deconvolute to spectra showed that the peak at 460 nm did not decay
on the 3-ms time scale. The features associated with the peak at 410 nm decayed on a 50 µs time
scale which corresponded roughly to a second-order rate constant of 2 * 105 M–1 s–1, taking into
account the concentration of HCl, although a real number could not be obtained due to the
overlapping nature of the spectral features. This rate constant suggested that 2 was approximately
as reactive with HCl as it was with PMe3 upon irradiation. At longer time scales such as 3 ms, a
feature grew in that had a peak at 366 nm, although it was difficult to determine whether this was
the true absorption maximum or if the true maximum lay further to the blue due to the lack of
white-light power beyond 400 nm. The appearance rate of this peak was on the order of 103 s–1.
An actual kinetic trace could not be obtained for this peak, again, due to the low intensity of the
white-light source in that region of the spectrum. This suggested an intermediate such as the HX
addition product or perhaps even the final photochemical product, 1.
2.5 Steady-state Bulk Photolysis Studies on 1 and 2
To determine if any bulk photochemistry was occurring during NSTA experiments on 2
when methylene chloride was used as a solvent, steady-state photolysis was performed on a
sample of 2 in methylene chloride. This experiment showed that 1 was produced by irradiation
(λ > 338 nm) of solutions of 2 in methylene chloride over the course of about 2 h. The photolysis
was followed by UV-vis spectroscopy. Including excess CO gas in the sample cell did not hinder
the progress of this reaction; it actually accelerated the reaction such that the conversion took
place in about 20 min. When this sample was irradiated further, 1 decomposed in a few hours,
16
going
a)
t = 0 - 60 mins
through
intermediate
2
an
with
unidentified
an
absorption
profile similar to 2. These results were
1
OD
further verified in THF and benzene
which showed that 2 was virtually
b)
2
1 atm CO
t = 0 - 150 min
unchanged over the course of an hour
t = 13.5 h
rapidly after addition of CO to the cell
in these two solvents, but decomposed
1
(Figure
300
400
Wavelength/ nm
2.6).
No
intermediate
500
resembling 1 was observed in these
Figure 2.8. Photolysis of 2 (λexc > 338 nm) in THF a) before
solvents. In the initial 10 min. of
and b) after addition of 1 atm CO.
irradiation of these samples some decomposition was observed but progressed no further during
the rest of the hour. This may be accounted for by photolabilized CO that escaped into the head
space of the sample cell and could not then back react with the putative open-coordination-site
species.
These results indicated that there must have been some equilibrium between the putative
open coordination site species and another species probably formed through ligand
rearrangement. This ligand rearranged species was in the minority but could be trapped out by
reacting with free CO in solution. For some reason when CO traps this species it was no longer
susceptible to photolabilization of the CO ligand at these irradiation wavelengths (Scheme 2.1).
Most
likely
dissociative
the
state
CO
was
higher in energy or had a
much
lower
OC Rh
P
0
P P
N N
hν
-CO
P
Rh
0
P
+CO
N
P
P
0
Rh
PPh3
N
P
P
0
P P
N N
Rh
0
PPh3
0
Rh
P Rh
P P
N
P
PPh3
P
P
N
+CO
molar
absorptivity than that of
2.
N
P
Scheme 2.1. Proposed mechanism for photolytic
decomposition of 2 in the presence of CO. “”
designates open coordination site.
17
N
P
0
P Rh
P P
N
0
Rh
P
N
P
PPh3
2.6 Experimental
All compounds were prepared in an N2-filled glove box or on an N2 Schlenk line, as
necessary, by previously described methods and determined to be pure by comparison with
known 1H,
19
F{1H}, and
31
P{1H} NMR spectra.12 Starting materials were obtained from Strem
Chemicals, Inc. (Newburyport, MA) in the case of Rh containing compounds and phosphines.
Other starting materials were obtained from Alfa-Aesar (Ward Hill, MA) and/or Aldrich
(Milwaukee, WI). Solvents used for UV-visible steady-state or transient absorption spectroscopy
were of UV Spectroscopic grade and were dried and then stored in degassed vacuum Strauss
flasks over NaK/Benzophenone for THF (Burdick & Jackson) from AlliedSignal (Muskegon,
MI) and benzene (Aldrich) (deep-purple solutions), and CaH2 for methylene chloride (Burdick &
Jackson). Solvents for the preparation of compounds were used appropriately after drying by the
usual methods.14
Time-resolved data were collected on a nanosecond laser instrument utilizing a Coherent
(Auburn, CA) Infinity XPO tunable laser (fwhm = 7 ns) as the source. This instrument was
previously described in part,15 some of which will be repeated here for completeness. The
Infinity Nd:YAG laser system consisted of an internal diode pumped, Q-switched oscillator,
which provided the seed pulse for a dual rod, single lamp, amplified stage. Sequential second and
third harmonics of 532 and 355 nm radiation were generated from the Nd:YAG 1064 nm
fundamental via Type I polarization and frequency mixing in tuned BBO crystals. The resultant
355-nm third harmonic was used to directly excite the sample after attenuation by a Newport
(Irvine, CA) 935-10 high-energy variable attenuator. Probe light was generated by a 75-W xenon
arc lamp powered by a PTI (Lawrenceville, NJ) LPS-1000 power supply and housed in a PTI
A1000 lamp housing. The lamp output was focused to a spot size of ~2.5 mm at the sample
collinear with the laser excitation via f/4 collimating and f/4 focusing lenses which was
refocused by f/4 focusing lens onto the entrance slit of an Instruments SA (Edison, NJ) Triax 320
monochromator. The signal wavelengths were dispersed by a grating possessing a blaze
wavelength of 500 nm and 300 grooves/cm and detected with a modified Hamamatsu R928 PMT
18
for single-wavelength kinetics or an Andor (Belfast, UK) DH520-25F-01 iCCD camera for
transient absorption spectra.
For kinetics the PMT was desensitized for absorption experiments by disabling 4 dynodes
and its response time was decreased by the addition of capacitors and larger resistors between the
remaining 6 dynodes. The output from the PMT was fed into a LeCroy (Chestnut Ridge, NY) 1
GHz 9384CM digital oscilloscope, which was triggered by a photodiode in the path of the laser
pulse (20 Hz repetition rate).
For transient absorption spectra, four absorption spectra were acquired sequentially and
processed to produce one transient spectrum, which was repeated typically 250 times and
averaged. The four spectra were recorded sequentially at a rate of 20 Hz under the conditions:
white-light off, excitation on; white-light on, excitation off; white-light on excitation on; and
white-light off, excitation off. The white-light shutter timing was controlled by a Stanford
Research Systems (SRS) (Sunnyvale, CA) DG-535 4-channel digital delay/pulse generator via a
Vincent Associates (Rochester, NY) UniBlitz T132 shutter driver/timer triggered by a
photodiode in the path of the laser pulse. A second SRS delay/pulse generator gated the iCCD,
which was triggered off the laser power supply Q-Switch Synch (20 Hz repetition rate). This
SRS delay/pulse generator additionally output to a second UniBlitz shutter driver/timer used as a
delay generator which output to the laser power supply Q-Switch Blank, reducing the laser
repetition rate from 20 to 10 Hz.
For both kinetics and spectra acquisition, oscilloscope operation, monochromator
operation, data storage, and data manipulation were managed by National Instruments driver
software (LabVIEW) incorporated into programs written at MIT. Communication between a Dell
Optiplex GX-1 computer and the instrumentation was achieved through an IEEE-488 (GPIB)
interface.
All transient absorption experiments were performed on samples contained in a 2-mm
exact-path length flow cell modified for high-vacuum use by the addition of a 100-mL solvent
reservoir separated from the cell by a high-vacuum PTFE valve. A typical sample contained 8.2
19
mg of rhodium complex and 25 mL of solvent. Addition of HCl, HBr, and PMe3 was
accomplished by transfer via the high-vacuum line from H2SO4 dropped on NaCl, Aldrich
lecture bottle, and previously-prepared vacuum pot, respectively.
Steady-state photolyses were completed using a 1000-W high-pressure Oriel Hg-Xe
lamp. The irradiation beam passed through cutoff filters to remove high-energy light and a
collimating lens prior to entering the sample chamber. Samples monitored by NMR were
prepared in Wilmad 528 NMR tubes adapted with a J. Young PTFE valve. Typically 15 – 20 mg
of a rhodium complex was used per sample to facilitate acquisition of NMR spectra. Samples
monitored by UV-visible spectroscopy were prepared in a cell equipped with a solvent reservoir
and a 1-cm clear fused-quartz cell (Starna Cells, Inc.). The two chambers were isolated from
each other by a high-vacuum PTFE valve and from the environment with a second high-vacuum
PTFE valve. A typical solution was 5 µM in rhodium complex. Bulk photolyses were performed
on stirred solutions contained in a quartz reaction vessel adapted for manipulations on a highvacuum manifold. All solvents were vacuum-transferred from previously dried and degassed
vacuum pots on a high-vacuum line and were subjected minimally to three additional freezepump-thaw cycles (10–5 torr) prior to irradiation. The THF vacuum pot contained a deep purple
solution of NaK/benzophenone in THF, and methylene chloride was stored in a vacuum pot over
CaH2. Both solvents were Burdick & Jackson spectroscopic grade solvents. Samples were
irradiated at 25 °C maintained by a circulating bath. UV-visible spectra were recorded on a
Spectral Instruments (Tucson, AZ) Model 440 spectrophotometer controlled by a Gateway 2000
P5/75 computer. 1H, 19F{1H}, and 31P{1H} NMR spectra were recorded on the Varian Mercury
300 spectrometer in the DCIF at MIT.
20
Chapter 3. Photochemical and Thermal Studies on the High-Valent Cycle
3.1 Introduction
In the course of investigations of HX photoreactivity with dinuclear rhodium compounds,
a second cycle for the production of H2 was identified. This cycle made use of the Rh2II,II / Rh20,II
two-electron redox couple. In theory, this high-valent cycle appeared more favorable than the
low valent cycle. This was for two reasons: 1) the crucial two-electron photoreduction was an
order of magnitude more efficient as indicated by the quantum yields of these processes; and 2)
the wavelength of light needed to accomplish the photoreduction was lower in energy.12
Some initial photochemical studies previously undertaken demonstrated that this method
only produced a third of the H2 that the low-valent cycle did before decomposing in
approximately the same time frame (1 – 2 days). The other result was that the steady-state
species during the photochemistry was not the Rh2II,II species, 3, as would have been expected by
analogy with the chemistry of the low-valent cycle. The steady state species was shown to be 4a,
Rh2dfpma3Cl2, a Rh20,II core with one chelating and two bridging PNP ligands. This was a novel
binding motif of the PNP ligands for Rh, although it had been observed for PNP ligands on a
diiridium core. This species was identified by its independent, thermal (non-photochemical)
preparation, by leaving Rh2dfpma3Cl2(CO) in vacuo (10–5 torr) for a few hours, and comparison
with the 19F NMR spectrum of the photochemical product. The 19F NMR indicated a PF2 moiety
axially bound to the Rh2 core, thus suggesting the “mono-chelating” structure. No crystal
structure was ever obtained to verify this assignment.
3.2 Thermal Synthesis of Rh2dfpma3Cl2, 4a
The observation of this new species in the high-valent photocatalytic cycle questioned the
notion that 3 was the species primarily responsible for the photoreduction. It was possible that
this cycle was going through a new Rh20,II/Rh20,0 couple and not the Rh2II,II/Rh20,II couple that
was originally presumed. To test this hypothesis, a more convenient synthesis of 4 was needed to
21
determine the photochemistry of this species alone. This was accomplished by allowing three eq.
of the dfpma ligand to react with one eq. of [Rh(COD)Cl]2. In this way, the desired product
could be obtained in over 70% yield as identified by the
19
F NMR spectrum and elemental
analysis. With this substance in hand, studies of its thermal and photochemistry were undertaken.
3.3 Performance of 4a as a Hydrogen-Generating Photocatalyst
Initial investigations examined the competence of 4 to perform photocatalytic H2
production under the same conditions as 3. When 3 was taken in a 0.1 M HCl/THF solution and
irradiated for 2 days only 3 – 5 eq. of H2 were obtained, as compared to 20 – 50 equivalents H2
obtained when using 3 as the precatalyst. While some catalytic activity was clearly present, 4
could not be the primary species responsible for H2 production in the case where 3 was used as
the precatalyst. While 4 was not a good example of a photocatalyst for hydrogen production, as a
new coordination motif, it might exhibit some interesting thermal and photochemistry.
3.4 Subsequent Thermal and Photochemical Reactivity of 4
From previous studies, it was known that 4 would not react thermally with HX.12 This
was somewhat surprising considering its putative vacant axial coordination site. This did not
necessarily preclude the possibility of it reacting thermally H2 adding two hydride ligands across
a M—M bond, reversibly, as was seen in the comparable Ir2 system.16 Unfortunately, no reaction
was observed when 4a was taken up in methylene chloride under > 1 atm H2 gas.
Photochemical investigations proved complex and led to the observation of relatively few
new phenomena. When 4a was irradiated in the presence of HX in a non-radical trapping solvent
such as methylene chloride the product obtained was 3 in approximately a 50% yield. This
contrasted to results found using THF as solvent. In this case, no reaction of the starting material
was observed excepting some photocatalysis and some photodegredation. This might be
indicative of the photochemical pathway of HX conversion for the upper cycle and help explain
why 4a was the main constituent in the steady state during bulk photocatalysis when initiating
22
with 3. It did not answer the question of why 4a by itself was much poorer in its ability to
produce hydrogen under the same conditions. The photochemistry of this molecule was typified
by undetermined decomposition pathways.
Furthermore, there were some indications that this substance could be a mixture of two or
more components despite passing elemental analysis for C, H, N, and P, for both the original and
the more recent thermal syntheses. This evidence included more peaks in the 1H NMR spectrum
than the two methyl resonances (possibly overlapping) that would be predicted by the putative
structure and that these peaks varied in relative intensity from sample to sample. Further
indications were that multiple spots were found on a tlc plate although decomposition on silica
has been implicated and the mixture does not elute on basic alumina in any solvent system.
3.5 Experimental
General considerations for thermal and photochemical synthesis were as stated in chapter
2. Elemental analysis was performed by QTI (Whitehouse, NJ).
Rh2dfpma3Cl2, 4a. In an N2-filled glovebox, [(COD)RhCl]2 (400 mg, 0.800 mmol) was
dissolved in 8 mL benzene in a 20-mL scintillation vial. To this a 2 mL solution of benzene
containing 406 mg of dfpma (2.43 mmol) was added slowly dropwise. After a few hours the
desired product precipitated out of solution. Adding hexanes facilitated additional precipitation
of product. The solid was filtered off and washed with hexanes yielding 440 mg (71%) of the
desired product.
19
F NMR compared exactly with samples prepared by previous methods
(photochemical and thermal) and elemental analysis was satisfactory in C, H, N, and P.
Reaction of 4a with H2. An NMR tube with a J. Young PFTE valve was charged with 15 mg of
4a. d2-Methylene chloride was added to the tube from a Strauss flask via vacuum transfer
methods. Then the tube was cooled to –78 °C and placed under 1 atm H2. The solution was
allowed to come to equilibrium for a half hour, sealed and warmed to room temperature. No
23
change in the 1H or 31P{1H} NMR spectrum was observed excepting a peak corresponding to H2
at δ = 4.6 ppm in the 1H over the course of 2 days. Heating the sample up to 50 °C affected no
change.
24
Chapter 4. Studies of a Mononuclear Hydrogen-Generating Photocatalyst
4.1 Introduction
Due to the previously unreported observation that Wilkinson’s catalyst, (PPh3)3RhCl, was
able to photocatalytically produce H2 under conditions similar to that of the binuclear
photocatalysts, studies to further investigate this result were undertaken. This system was
interesting in that PPh3 could be used as a Cl2 acceptor to form dichlorotriphenylphosphorane,
PPh3Cl2. This was not possible in previous systems in which σ-donating ligands would block
axial ligation sites and inhibit catalytic activity. In this system, open coordination sites were not a
problem as triphenylphosphine was long known to dissociate readily from the core in solution
due to steric effects, generating a small but highly reactive amount of three-coordinate species.17
Since this was not necessarily a radical trapping mechanism the possibility also existed that this
system eliminates Cl2 as well as H2 coming one step closer to an energy conversion system,
despite the need for a trap. Furthermore this system suggested that the photocatalysis could be
accomplished by a monometallic system which would mean that pre-organized cooperativity was
not a requirement for this multielectron photoreductive chemistry. From an industrial standpoint,
compounds of this type are already produced in large amounts for catalytic (thermal)
hydrogenation as well as for industrial and academic research.
4.2 General Observations of the Mononuclear System
In a typical experiment 15 mg of Wilkinson’s catalyst was dissolved in 0.75 mL
deuterated solvent (usu. CD2Cl2) along with 85 mg PPh3 (~ 20 eq.) in an NMR tube with a J.
Young PTFE valve. Acid (HCl) was then added by typical high vacuum line procedures to make
a 2 M HCl solution. The sample was irradiated (λ > 338 nm, wavelengths below 360 nm were
reduced by absorption of the NMR tube) for ~3 days during which it was monitored by NMR
(1H and
31
P) to determine PPh3Cl2 content and observe the rhodium species present. These
experiments typically showed some H2 production by NMR as well as concomitant production of
25
PPh3Cl2. Also observed was the rapid decomposition of peaks indicative of Rh—P bonds. No Rh
metal was observed to precipitate out of solution so it was assumed that P remained bound to Rh
in a variety of products all too low in concentration to be readily observed in the
31
P NMR
experiment. In a typical experiment 2 – 5 eq. of H2 were collected and measured by the Toepler
pump method. In one instance this was able to be correlated to 75% of the PPh3Cl2 observed.
This lent some credence to the chemical equation:
PPh3
+ 2 HCl
hν
(PPh3)3RhCl
PPh3Cl2
+
H2
By no means was it confirmation of the stoichiometry indicated. The above reaction did not
proceed at all in the absence of Rh compound. The steady state photochemical constituent could
not be identified by NMR. Instead a spectrum broadened by fluxional effects was obtained.
4.3 Variable Temperature NMR of the HCl Addition Step
To more carefully characterize the cycle, HCl was added to Rh(PPh3)3Cl in an attempt to
produce the presumed and previously characterized intermediate Rh(PPh3)3HCl2. Despite many
attempts with different sources of HCl and Rh(PPh3)3Cl, this species could not be cleanly
generated. What was formed produced a 1H and
31
P{1H} NMR spectrum that was fluxional in
nature. To deconvolute these spectra variable temperature NMR experiments were undertaken at
both lower and higher temperature. The results of those experiments were inconclusive. The
broad peaks seen at room temperature did separate out and sharpen as the temperature was
lowered, but these new splitting patterns could not account for any particular structure or
combination of structures postulated to be present. The splitting pattern for 31P was assigned as
two doublet of triplets and one doublet of doublets (δ = 46.2, 45.2, 43.2) in a 3:3:4 integration
ratio. In all cases the larger doublet around 136 Hz was assigned to the 1JRh—P coupling and the
smaller doublets and triplets in the 17 – 20 Hz range were assigned to the 2JP—P coupling. The 1H
spectrum at 213 K exhibits what look like two doublets of triplets in the hydridic region (δ = 16.5, -16.9) with coupling constants around 13 – 20 Hz. The two resonances integrate at a 3:2
26
ratio. The analysis of the 1H spectrum was very tentative due to broadened resonances persisting
at 213 K.
4.4 Determination of 31P Spin-Lattice Relaxation Times
To observe reliable integration of the
31
P NMR resonances of PPh3 and PPh3Cl2 the
relaxation time of each was determined. Accurate integration was essential for comparison of the
PPh3Cl2 production as measured by 31P NMR with the amount of H2 produced as determined by
the Toepler pump method. An inversion recovery type experiment was performed on an NMR
sample containing both PPh3 and PPh3Cl2 so that both could be determined at once. A PRESAT
experiment was used in which the sample is saturated by a 2 ms pulse and then allowed to relax
for an arrayed delay period after which a π/2 pulse is used to observe the amount of decay that
occurred. Integrating the resonances in question and determining the fit of the exponential decay
versus time revealed a spin-lattice relaxation time, T1, of 1.7 s for PPh3 and 11.8 s for PPh3Cl2.
This meant that for accurate integration of these two signals the delay between experiments in a
typical π/2 acquisition should be at least 60 s (> 5T1). This was the delay used for all
experiments in which the integration intensity of PPh3 and PPh3Cl2 were compared.
4.5 Experimental
General considerations for thermal and photochemical syntheses were the same as
chapter 2. In a typical experiment demonstrating photocatalysis, a Wilmad NMR tube with J.
Young PTFE valve was charged with 15 mg (PPh3)3RhCl (16 µmol) and 90 mg PPh3 (340
µmol), recrystalized. To this 0.75 mL of CD2Cl2, previously dried over CaH2 and degassed, was
added via standard high-vacuum transfer techniques. Additionally, 1.4 mmol HCl produced by
dropping H2SO4 on NaCl was added via high-vacuum techniques. The tube was warmed to room
temperature and irradiated (λ > 338 nm) for a total of 2 to 4 days. A 1H and 31P NMR spectrum
was obtained every few hours to follow the production of PPh3Cl2. After irradiating the sample
the H2 was collected and quantified using a Toepler pump.
27
In the case of variable temperature NMR and T1 determination, the NMR experiments
were performed on a Varian Inova 500 spectrometer in the DCIF at MIT. In the case of the T1
determination, the PRESAT pulse sequence, a variation on the inversion recovery experiment,
was employed, using an array of delay times to determine the spin-lattice relaxation time
dependence.
28
References
1) Ciamician, G. Science 1912, 36, 385-394.
2) Lehn, J. M.; Sauvage, J. P. Nouv. J. Chim. 1977, 1, 449-451.
3) Kirch, M.; Lehn, J. M.; Sauvage, J. P. Helv. Chim. Acta 1979, 62, 1345-1384.
4) Mann, K. R.; Lewis, N. S.; Miskowski, V. M.; Erwin, D. K.; Hammond, G. S.; Gray, H. B. J.
Am. Chem. Soc. 1977, 99, 5525-5526.
5) Mann, K. R.; Bell, R. A.; Gray, H. B. Inorg. Chem. 1979, 18, 2671-2673.
6) Dulebohn, J. I.; Ward, D. L.; Nocera, D. G. J. Am. Chem. Soc. 1988, 110, 4054-4056.
7) Dulebohn, J. I.; Ward, D. L.; Nocera, D. G. J. Am. Chem. Soc. 1990, 112, 2969-2977.
8) Nixon, J. F. J. Chem. Soc. A 1968, 2689-2692.
9) Spessard, G. O.; Miessler, G. L. Organometallic Chemistry; Prentice-Hall: Upper Saddle
River, New Jersey, 1997.
10) Heyduk, A. F.; Nocera, D. G. Science 2001, 293, 1639-1641.
11) Sigal, I. S.; Gray, H. B. J. Am. Chem. Soc. 1981, 103, 2220-2225.
12) Heyduk, A. F. Two-Electron Mixed-Valence Complexes: Small Molecule Activation and
Photocatalytic Hydrogen Production. Ph.D. Thesis, Massachusetts Institute of Technology,
Cambridge, MA, 2001.
13) Heyduk, A. F.; Macintosh, A. M.; Nocera, D. G. J. Am. Chem. Soc. 1999, 121, 5023-5032.
14) Armarego, W. L. F.; Perrin, D. D. Purification of Laboratory Chemicals; 4 ed.; ButterworthHeinmann: Oxford, 1996.
15) Rudzinski, C. M.; Young, A. M.; Nocera, D. G. J. Am. Chem. Soc. 2002, 124, 1723-1727.
16) Heyduk, A. F.; Nocera, D. G. J. Am. Chem. Soc. 2000, 122, 9415-9426.
17) a) Meakin, P.; Jesson, J. P.; Tolman, C. A. J. Am. Chem. Soc. 1972, 94, 3240-3242. b)
Halpern, J.; Okamoto, T.; Zakhariev, A. J. Mol. Catal. 1976, 2, 65-69. c) de Croon, M. H. J.
M.; van Nisselrooij, P. F. M. T.; Kuipers, H. J. A. M.; Loenen, J. W. E. J. Mol. Catal. 1978,
4, 325-335.
29
Curriculum Vitae
David James Krodel was born to Jim and Anne Krodel in Norwich, Connecticut on
August 7, 1978, the first of their three children. He spent his entire childhood in Salem,
Connecticut where he attended Salem Elementary School. After the eighth grade, David entered
high school at East Lyme High School in East Lyme, Connecticut where he was affected greatly
by the superb faculty. He was especially influenced by his English teacher/drama director Mr.
James Warykas, his choral director Ms. Mary Ann Liniak-Bodwell, and his chemistry teacher
Mr. T. Carl Reichard. After finishing in June 1996 as salutatorian at ELHS, he enrolled at
Northwestern University in Evanston, Illinois. After a brief stint as a theater student, he majored
in chemistry after taking organic chemistry from Professor Joseph B. Lambert. Professor
Lambert took him under his tutelage as an undergraduate research student and with his guidance
studied the synthesis of oligosilanes designed for photochemical electron transfer rate studies.
He completed his Bachelor of Arts in chemistry summa cum laude with departmental honors in
June 2000, entering the Massachusetts Institute of Technology in Cambridge, Massachusetts as a
graduate student in inorganic chemistry the following fall. He quickly gravitated toward
Professor Daniel G. Nocera who intrigued him not solely for his stimulating multidisciplinary
program in physical and inorganic chemistry. After two educational and self-revealing years in
graduate school, he has decided to finish his Master’s Degree with Professor Nocera and enter
the work force, where he hopes to continue to learn about chemistry, science, life, and himself.
30